Golf balls are generally classified into rubber thread type golf balls and solid type golf balls. The rubber thread type golf ball is formed such that a core of a rubber thread wound up into a sphere is covered with a balata cover. The solid type golf ball is formed such that a spherical core made of hard rubber having a high restitution coefficient is covered with a resin cover excellent both in tear resistance and in wear resistance. The latter type golf ball is overwhelmingly popular with golfers because it is excellent in carrying distance.
However, the solid type golf ball has problems, for example, such that it gives a relatively hard feeling at the time of hitting and that it is difficult to give it the required amount of a spin by hitting with a short iron. Thus, there is still room for improvement in the solid type golf ball. As measures against these problems, the following examples of improvement are known
A first one of the examples is a structure in which a relatively thin intermediate layer made of a resin having lower hardness than a cover is disposed between the core and the cover, or two intermediate layers made of such a resin are disposed as occasion demands.
A second one is applied to the above mentioned-structure in which a resin intermediate layer is disposed between the core and the cover. For example, as described in JP-A-10-216270, small protrusions are provided on the whole surface of the intermediate layer (hereinafter referred to as "inner cover") and the outside of the inner cover is covered with the cover (hereinafter referred to as "outer cover"). As a structure similar to this, there is also a proposal of a structure in which small protrusions are provided on an inner circumferential surface of the outer cover so as to protrude into the inner cover. Also in this case, a resin material having lower hardness than the outer cover is generally used for the inner cover. When such protrusions are provided on the inner cover, the height of the protrusions is often set to be slightly smaller than the thickness of the outer cover. When such protrusions are provided on the outer cover side, the height of the protrusions is often set to be slightly smaller than the thickness of the inner cover. As a result, the outer cover material is interposed between adjacent protrusions extended from the inner cover or the inner cover material is interposed between adjacent protrusions extended from the outer cover. Hence, when the golf ball is viewed radially, a third layer having mixed properties of the outer cover and the inner cover is substantially present at the location of the small protrusions.
Such a multi-structure solid type golf ball constituted by a core, an inner cover and an outer cover is generally formed as follows. That is, a rubber material is put into a mold and vulcanized into a spherical core. Then, the spherical core is shifted into an inner cover-forming mold and a thermoplastic resin is injected into the mold. Thus, the outside of the core is covered with the thermoplastic resin so that an inner cover is formed. Then, the core covered with the inner cover is further shifted into an outer cover-forming mold and a similar thermoplastic resin is injected into the mold. Thus, the formation of the golf ball exhibiting the external appearance as a final product is completed.
In the golf ball thus formed, faces between adjacent constituent layers are in tight contact with one another but not adhesively bonded to one another. Particularly when such a ball in which faces between laminated layers are not adhesively bonded to one another is hit by a club having a large loft angle, there is a defect that it is difficult to give an expected spin to the ball because of loss occurring in shear stress generated in the ball. The tendency to a lower spin based on the loss in the shear stress is exhibited more remarkably in a portion near the surface of the ball and between the inner cover and the outer cover. In this respect, the interposition of the aforementioned small protrusions between the inner cover and the outer cover can improve mechanical connection therebetween but is not satisfactorily reflected in spinning characteristic.